The first case of female-to-male sexual transmission of the Zika virus has been documented in New York City, raising the prospect that the disease could spread more widely beyond the countries where it is already endemic and largely transmitted by mosquitoes.

For months, there has been growing concern about the dangers of sexual transmission, but until now the virus has been thought to pass only from men to women or between two men.

“This represents the first reported occurrence of female-to-male sexual transmission of Zika virus,” said a report issued on Friday by the Centers for Disease Control and Prevention and the New York City Department of Health and Mental Hygiene.

The evidence of a previously undocumented means of transmission is the latest twist in a viral outbreak that continues to baffle and surprise the world’s leading experts. It is forcing officials to rethink, once again, the guidance for health care providers and the general public on how to limit the danger of infection, as the pool of those who could be at risk widens.

Much about how the virus works is a mystery, and it remains challenging to detect; 80 percent of those infected show no symptoms. For those who do get sick, the illness is often mild, and there is no treatment.

But Zika can pose a dire risk to pregnant women. It targets developing nerve cells in fetuses and can lead to a birth defect called microcephaly, in which babies are born with abnormally small heads and brain damage. It may also cause developmental problems after birth.

Zika is primarily transmitted by the yellow fever mosquito, Aedes aegypti, which thrives in warm, tropical climates. But 11 countries have documented cases of sexual transmission from a man to a woman. Among the 1,130 people who have received a Zika diagnosis in the continental United States, including 320 pregnant women, the C.D.C. has reported 15 cases of sexual transmission.

In a reflection of the urgency of the situation, White House officials joined with congressional leaders and public health officials this month to denounce the failure of lawmakers to provide much-needed funding to combat the virus. The legislative session in Congress ended Thursday with lawmakers failing to provide money to fight it.

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“The more we learn about Zika, the more concerned we are,” Dr. Thomas R. Frieden, the director of the C.D.C., said during a recent conference call with reporters.

There have been at least seven children born with birth defects and five pregnancy losses related to Zika in the United States. The lifetime cost of care is estimated to be $10 million for each sick child.

“Each case is a tragedy,” Dr. Frieden said. “A child that may never walk or live independently.”

The New York case is the first in which a man was infected by a woman, and it raises the prospect that other men — with no travel history to Zika-affected areas and no reason to suspect that they might have the virus — could become infected and pass the virus on, creating a new chain of transmission.

In the report, researchers found that a man, who was in his 20s and did not travel outside the United States during the year before his illness, contracted the virus after one instance of vaginal intercourse, without a condom, with a woman who had recently returned from a country where the virus is endemic.

Dr. Mary T. Bassett, New York City’s health commissioner, said that there were several factors in this case that might have raised the risk of infection: The man was uncircumcised, the woman was in the early stages of her illness when her viral load was high, and she was also at the beginning of her menstrual cycle.

The woman, described as being in her 20s and not pregnant, had sex with her partner the day she returned to the city. The report does not name the country she visited, but the virus is now widespread in nearly 50 countries throughout South America and the Caribbean.

“She reported having headache and abdominal cramping while in the airport before returning to N.Y.C.,” the report said. The next day she developed a number of symptoms associated with Zika, including fever, fatigue, a rash, back pain, swelling of the extremities, and numbness and tingling in her hands and feet.

She reported that her period, which began that day, was also heavier than usual.

Her primary care physician sent blood and urine samples to the city and state health department laboratories for testing. The tests detected the virus but not antibodies to it, which suggested that she was newly infected; it takes four or five days for the body to begin producing antibodies.